What causes hemifacial spasm?

Causes of Hemifacial Spasm

Hemifacial spasm is most commonly caused by vascular compression of the facial nerve (CN VII), typically by an ectatic vessel at or near the root exit zone from the brainstem, specifically within the centrally (oligodendrocyte) myelinated portion of the nerve. 1, 2

Primary Etiology: Neurovascular Compression

The overwhelming majority of hemifacial spasm cases result from pulsatile compression of the facial nerve by blood vessels in the posterior fossa. 2, 3

Key anatomical considerations:

• The compression typically occurs at the root exit zone (REZ) where the facial nerve exits the brainstem at the pontomedullary sulcus 1, 2
• The centrally myelinated segment (oligodendrocyte-myelinated rather than Schwann cell-myelinated) is particularly vulnerable to vascular compression 1
• Distal cisternal compression can also occur, with the anterior inferior cerebellar artery (AICA) being the offending vessel in 84% of distal compression cases 4
• Neurovascular contact is identified in 83-100% of cases on high-quality imaging, though this finding should be considered supportive rather than diagnostic 2

Secondary Causes

While vascular compression accounts for the vast majority of cases, secondary causes must be excluded and include:

Brainstem pathology affecting the facial nerve nucleus or fascicles: 1, 2

• Infarction
• Vascular malformations
• Tumors
• Multiple sclerosis

Cerebellopontine angle lesions: 1, 2

• Facial and vestibular schwannomas
• Meningiomas
• Epidermoid cysts
• Paragangliomas

Temporal bone pathology: 1, 2

• Cholesteatomas
• Intrinsic bone tumors
• Trauma
• Inflammatory middle ear disease

Extracranial causes: 1, 2

• Parotid tumors
• Skull base carcinomas and sarcomas
• Inflammatory disease of the skull base

Pathophysiological Mechanism

The spasm-related electromyographic activity is generated by ephaptic transmission (abnormal cross-talk between nerve fibers) due to local demyelination at the entry zone of the facial nerve root, likely caused by chronic pulsatile compression from the offending vessel. 5 This "nerve origin hypothesis" explains why the spasms are peripherally induced despite originating from a central compression point. 5

Diagnostic Imaging Recommendations

MRI with 3D heavily T2-weighted sequences and MR angiography is the imaging modality of choice to characterize vascular loops potentially compressing the facial nerve and to exclude secondary causes. 6, 2, 7

Important imaging caveats:

• High-resolution thin-cut sequences through the entire course of CN VII are essential 7
• Pre- and post-contrast imaging should be obtained to identify and characterize lesions 7
• 3T imaging provides improved visualization of the facial nerve and surrounding perineural vascular plexus 7
• MRI findings of neurovascular contact are supportive but not diagnostic when selecting surgical candidates 6, 2